With the "commendation" system and vastly larger weapon & equipment inventories; why would you even bother to advance your Faction reputation?
Only for the armor bonuses to Strength, Discipline and Intellect?
Commendations ensure you must progress in Vanguard and Crucible to buy Legendary items if RNG isn't kind to us.
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If you don't need to buy gear from the vanguard you can just use your factions to get crucible commendations and have a chance at getting a cool shader that's why I rank up factions anyway